#0023: Repairing short circuit damage within a ribbon cable

Preamble

Sometimes you might come across damage within a ribbon cable similar to the example. The minor burn damage on the example featured was done by a liquid causing a short circuit between two exposed copper pads. As it burned, it created a break between an exposed pad, and it’s respective trace. Cutting the circuit in the process. The short also caused some of the other pads to oxidise, and some minor burning of the ribbon cable’s plastics. This will be a quick tutorial on repairing such a fault.

Please note: I have no images before the initial cleaning and prepping stage. This is because I was halfway through this repair, when I decided that it may be good to document it.

Tools and materials:

• scalpel
• tweezers
• isopropyl alcohol
• cotton earbuds
• soldering iron
• lead solder
• copper strand from a wire
• side cutters
• multimeter

Step 1: Identifying and logging the damages

This should always be a first step before attempting a repair. The reason for this is that the initial pre-work cleaning, is likely to clean away a lot of contextual clues about the location and severity of all the damages. A good visual inspection and initial assessment can save time later on, due to not having to track down any circuit damage that got masked or hidden by cleaning.

Step 2: Cleaning the local area

First thing I did after identifying the location of any potential damage I wanted to repair, was to clean it. In this case I needed to first scrape off the more obvious patches of oxidation and burnt/melted materials using a scalpel. Then thoroughly clean off the pads of the ribbon cable using isopropyl alcohol and a cotton earbud. I paid special attention to the tiny burn hole next to the most damaged pad, making sure to remove any conductive materials from it by scraping it out thoroughly.

Step 3: Test to confirm faults

Using a multimeter in continuity mode, I tested for continuity between the pads surrounding the burnt spot. Perhaps it still contained conductors (such as pieces of the broken pad) that may cause a future short. After being satisfied that it did not; I focused on the particular pad that sustained the most damage, and tested for continuity across this pad and it’s respective trace to see if it was still connected. It was not. All other pads had continuity with their respective traces.

Step 4: Fixing confirmed faults

After identifying only a single fault; this being a break between a pad and it’s trace. I moved to repair it. Firstly I endeavoured to bridge the gap by just tinning across it from the pad to it’s trace. The thought process was that that the mere mass of the solder itself would be enough to bridge the tiny gap. It did not. After that initial failure, I decided that I required a bridging medium; something for the solder to adhere to. In this case I decided to use a single copper strand from a wire.

Using a scalpel, I removed some insulation from the ribbon cable trace above the broken pad. This was in order to have something on that side to comfortably solder the copper strand to. After which, I soldered the wire whilst using a pair of tweezers to hold the tiny copper strand down in place.

Step 5: Cleaning up after a repair

The next step involves cleaning up any messes I might’ve caused during the repair. In this case, whilst trying to initially tin the broken pad, I also tinned the neighbouring pads accidentally. In trying to remove the bulk of the solder, I caused further damage by starting to burn the plastic that the pads are set into. In the end I decided to just leave it be. The repair needed to be functional, not aesthetically pleasing. I did consider using a desoldering braid to remove all the solder, however I was very likely to cause more damage trying, so I opted to just leave it be.

In the end the after-fix clean consisted of just clipping off the excess wire with a side cutter and cleaning off any flux residue with an isopropyl dipped cotton earbud.

Step 6: Testing the repair

Next. I performed a quick continuity test on the repair with a multimeter; both testing for continuity across the pad to it’s trace; and testing for a lack of continuity across neighbouring pads. These tests take basically no time to do and can set my mind at ease. I do these types of tests even when a visual inspection indicates that it’s not necessary.

The real test is putting the device back into service and seeing if it functions as expected. In this case the ribbon cable that I repaired was from a laptop’s integrated keyboard. Although the ribbon cable fitted more snugly into it’s receptacle or socket than I wanted. It still fitted and functioned properly. The keyboard was fully functional after this repair. Huzzah!

Closing thoughts

I apologise if this article came off as a little patronising; especially given the quality of the example repair, the missing “before” picture, and the fact that it contains mistakes front and centre. Generally, I find step-by-step guides like this one difficult to write, without sounding either needlessly pedantic or excessively didactic. Anyway, even if the exact specifics are not useful to you, I hope the general steps would be. Identify. Clean. Test. Fix. Clean. Test. Then Profit. That is if it works, if not: then go back to testing for faults.

Thank you for reading.